Method for forming multi-layer metallic coating film

ABSTRACT

A method for forming a multi-layer metallic coating film, the method being a 3-coat system comprising the steps of applying the following coating compositions (A), (B) and (C) in this order to a metal substrate coated with a cationically electrodepositable coating composition; (A) an organic solvent-based thermosetting metallic first base coating composition which comprises a neutralized carboxyl-containing resin having an acid value of 5 to 100 mg KOH/g, an amino resin and a metallic pigment, the composition having a substrate-hiding power corresponding to up to 3% transmittance of light at a wavelength of 400 to 700 nm in a 15 μm-thick cured coating film, (B) an aqueous thermosetting second base coating composition which has a transparency corresponding to 10 to 95% transmittance of light at a wavelength of 400 to 700 nm in a 15-μm thick cured coating film, and (C) an organic solvent-based thermosetting clear coating composition. The method of the invention gives a multi-layer metallic coating film on a metal substrate, the film being improved in esthetic property, surface smoothness and so on.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a novel method for forming amulti-layer metallic coating film.

BACKGROUND ART

A metallic coating film sparkles when there occur reflections from lightrays incident on a metallic pigment in the coating film. Metalliccoating films have an esthetically attractive appearance of their ownwith wide variations as in color, and are chiefly applied to metalarticles such as automobiles, motor cycles and the like.

A 3-coat 1-bake coating method for forming a metallic coating film isknown from Japanese Examined Patent Publication No.25076/1992, themethod comprising applying an organic solvent-based thermosettingcoating composition comprising a neutralized carboxyl-containing resinhaving an acid value of 5 to 100 mgKOH/g and an amino resin, adjustingthe coating layer to a specific viscosity range, applying an aqueousthermosetting metallic coating composition, applying an organicsolvent-based thermosetting clear coating composition, and heating thecoatings for curing at the same time.

According to the foregoing conventional coating method, the first layerof organic solvent-based thermosetting coating composition is adjustedto a specific viscosity range while the second layer of aqueousthermosetting metallic coating composition is formed on theviscosity-adjusted first layer, whereby advantageously the amount of theorganic solvent can be reduced and the unevenness of metallic effect canbe prevented without degrading the esthetic property or surfacesmoothness of coating film which would occur because of surfaceroughening and sagging at varied humidities.

However, the foregoing conventional method has drawbacks that theobtained metallic coating film appears flat and deficient in depthfeeling, and is not always satisfactory in distinctness-of-image gloss,namely insufficient in esthetic property.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a novel method offorming a multi-layer metallic coating film which retains theabove-mentioned advantages of conventional coating methods but is freeof their drawbacks.

Another object of the invention is to provide a novel method capable offorming a multi-layer metallic coating film on a metal substrate, thefilm being enhanced in depth feeling and in distinctness-of-image gloss,consequently improved in esthetic property and surface smoothness.

Other features of the invention are apparent from the followingdescription.

According to the present invention, there is provided a method forforming a multi-layer metallic coating film, the method beingcharacterized by a 3-coat system comprising the steps of applying thefollowing coating compositions (A), (B) and (C) in this order to a metalsubstrate coated with a cationically electrodepositable coatingcomposition; (A) an organic solvent-based thermosetting metallic firstbase coating composition which comprises a neutralizedcarboxyl-containing resin having an acid value of 5 to 100 mgKOH/g, anamino resin and a metallic pigment, the composition having asubstrate-hiding power corresponding to not higher than 3% transmittanceof light at a wavelength of 400 to 700 nm in a 15 am-thick cured coatingfilm, (B) an aqueous thermosetting second base coating composition whichshows a transparency corresponding to 10 to 95% transmittance of lightat a wavelength of 400 to 700 nm in a 15-pm thick cured coating film,and (C) an organic solvent-based thermosetting clear coatingcomposition.

The present inventors conducted extensive research to overcome thedrawbacks of conventional methods while retaining the above-mentionedprior art advantages, and obtained the following novel findings.

(1) When the 3-coat system is conducted which comprises applying (A) thefirst base coating composition having the above-specified hiding power,(B) the second base coating composition having the above-specifiedtransparency and (C) the clear coating composition in this order, thereis formed a multi-layer metallic coating film which is improved in depthfeeling and distinctness-of-image gloss and therefore superior inesthetic property.

(2) When the second base coating composition (B) is applied, theneutralized carboxyl-containing resin in the first base coatingcomposition (A) exhibits a high water absorbing capacity and absorbswater from the coating composition (B) on the composition (A), resultingin rapidly increased viscosity of the composition (B). The film flaws ofsagging at a high humidity and surface roughening at a low humidity canbe avoided owing to the foregoing feature and a high wettability of thecomposition (B) toward the composition (A). Consequently the amount ofthe organic solvent to be used can be reduced without degradation ofesthetic property or surface smoothness of coating film due to thechange of humidity.

(3) The metallic first base coating composition (A) is in an organicsolvent form so that there is no likelihood of producing the unevennessof metallic effect.

The present invention was completed based on these novel findings.

The method for forming a metallic coating film according to the presentinvention is described below in more detail.

Metal substrates to be coated by the method of the invention are thosecoated by cationic electrodeposition. Useful metal substrates are notlimited insofar as they are those which can be coated by cationicelectrodeposition. Specific examples of useful metal substrates arethose of iron, copper, aluminum, tin, zinc or like metals, alloyscontaining such metals, metal parts plated with such metals, thosecoated with such metals by vapor deposition, bodies of automobiles,trucks, motor cycles, buses or the like having such metal parts,electric appliances, etc. These metal substrates are preferably thosesubjected to chemical conversion treatment with phosphate, chromate orthe like.

The cationically electrodepositable coating compositions to be appliedto metal substrates are not limited and include various types ofcationically electrodepositable coating compositions heretofore known,such as coating compositions comprising as a main component an aqueousresin, e.g. an epoxy resin, acrylic resin, polybutadiene resin or thelike which is modified with an amino group-containing compound andneutralized with a neutralizing agent, optionally the compositioncontaining a crosslinking agent such as blocked polyisocyanate,alicyclic epoxy resin or the like, a coloring pigment, an anticorrosivepigment, an extender pigment, a hydrophilic organic solvent or the like.Examples of useful neutralizing agents are acetic acid, hydroxylaceticacid, propionic acid, butylic acid, lactic acid, glycine and likeorganic acids, and sulfuric acid, hydrochloric acid, phosphoric acid andlike inorganic acids.

The cationically electrodepositable coating composition is diluted withdeionized water or the like to a solids content of about 5 to about 40%by weight and is applied at a pH range of 5.5 to 8.0 in the conventionalmanner. Then the coated substrate is heated to a temperature of about140 to about 210° C. for curing. A preferred film thickness is about 10to about 60 4m when cured.

If necessary, an intercoat composition may be applied to the coatinglayer of cationic electrodepositable composition. Suitable intercoatcompositions include, for example, thermosetting coating compositionscomprising a base resin, a crosslinking agent and a solvent as maincomponents and optionally additives for coating compositions such ascoloring pigments, extender pigments and the like.

Base resins to be incorporated in the intercoat composition include, forexample, acrylic resins, polyester resins and the like which have acrosslinkable functional group. Useful crosslinking agents include, forexample, melamine resins, urea resins, polyisocyanate compounds andblocked polyisocyanate compounds. Useful solvents are, for example,organic solvents and water.

The intercoat composition may be applied to a substrate covered with acationically electrodepositable coating composition by electrostaticcoating, air spray, airless spray or other coating methods. A preferredintercoat thickness is 10 to 50 Am when cured. The intercoat layer iscured by crosslinking when heated at about 100 to about 150° C.

The metallic first base coating composition (A) to be used in the methodof the invention is applied to the cured surface of electrodepositedcomposition or intercoat composition layer on the metal substrate. It isan organic solvent-based thermosetting coating composition having asubstrate-hiding power corresponding to up to 3% transmittance of lightat a wavelength of 400 to 700 nm in a cured coating film of 15 Mmthickness (which may be hereinafter called simply "lighttransmittance"). The first base coating composition (A), coupled withthe second base coating composition (B) having a transparencycorresponding to a light transmittance of 10 to 95% and the clearcoating composition (C), is capable of imparting markedly improved depthfeeling and distinctness-of-image gloss to the coating film, resultingin the formation of an esthetically superior multi-layer metalliccoating film. The first base coating composition (A) allows the secondbase coating composition (B) to form a uniform and smooth-surfacedcoating layer without influence from atmospheric humidity.

The first base coating composition (A) to be used in the inventioncontains as a base resin a carboxyl-containing resin having an acidvalue of 5 to 100 mgKOH/g. The carboxyl-containing resin useful hereincan be any of conventional various resins which can fulfil suchrequirement. Typical examples of carboxyl-containing resins are givenbelow.

(i) Typical examples include acrylic resins and vinyl resins prepared bycopolymerizing at least one of carboxyl-containing polymerizableunsaturated monomers, optionally at least one of hydroxyl-containingpolymerizable unsaturated monomers and other polymerizable unsaturatedmonomers, the resins preferably having a number average molecular weightof about 5,000 to about 40,000.

Preferred examples of the carboxyl-containing polymerizable unsaturatedmonomer are α,β-ethylenically unsaturated carboxylic acids such asacrylic acid, methacrylic acid, maleic acid, itaconic acid, crotonicacid and the like.

Examples of the hydroxyl-containing polymerizable unsaturated monomersto be optionally used are hydroxyalkyl (having 2 to 8 carbon atoms)esters of (meth)acrylic acids such as 2-hydroxyethyl (meth)acrylate,2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate andhydroxybutyl (meth)acrylate; monoesters of polyether polyols such aspolyethylene glycol, polypropylene glycol or polybutylene glycol withunsaturated carboxylic acids such as (meth)acrylic acid; monoethers ofsuch polyether polyols with hydroxyl-containing unsaturated compoundssuch as 2-hydroxyethyl (meth)acrylate; addition reaction products ofα,β-unsaturated carboxylic acids with monoepoxy compounds such as"Cardula E1O" (trade name, product of Shell Chemical Co., Ltd.) orα-olefin epoxide; addition reaction products of glycidyl (meth)acrylatewith monobasic acids such as acetic acid, propionic acid,p-t-butylbenzoic acid and fatty acids; monoesters or diesters of acidanhydride group-containing unsaturated compounds such as maleicanhydride and itaconic anhydride with glycols such as ethylene glycol,1,6-hexanediol and neopentyl glycol; hydroxyalkyl vinyl ethers such ashydroxyethyl vinyl ether; chlorine- and hydroxyl-containing monomerssuch as 3-chloro-2-hydroxypropyl (meth)acrylate; allyl alcohol and soon. Examples of other polymerizable unsaturated monomers are alkyl orcycloalkyl (having 1 to 24 carbon atoms) esters of (meth)acrylic acidssuch as methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl(meth)acrylate, i-butyl (meth)acrylate, t-butyl (meth)acrylate,cyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl(meth)acrylate, stearyl (meth)acrylate and decyl (meth)acrylate;(meth)acrylamides such as (meth)acrylamide, N-methyl (meth)acrylamide,N-ethyl (meth)acrylamide, diacetone acrylamide, N-methylol(meth)acrylamide and N-butoxymethyl (meth)acrylamide; glycidylgroup-containing vinyl monomers such as glycidyl (meth)acrylate,glycidyl (meth)acrylamide and allyl glycidyl ether; vinyl monomers suchas styrene, vinyltoluene, vinyl propionate, a-methylstyrene, vinylacetate, (meth)acrylonitrile, vinyl pivalate and "VeoVa" monomer (tradename, product of Shell Chemical Co., Ltd.).

(ii) Typical examples of the carboxyl-containing resin further includeoil-free polyester resins prepared by condensing an alcohol componentand an acid component; and oil-modified polyester resins prepared byreacting an alcohol component, an acid component and an oil component,these resins preferably having a number average molecular weight ofabout 500 to about 10,000.

The alcohol component can be a polyhydric alcohol or optionally may beused in combination with a monohydric alcohol, monoepoxy compound or thelike. Examples of polyhydric alcohols are glycols such as ethyleneglycol, propylene glycol, diethylene glycol, trimethylene glycol,tetraethylene glycol, triethylene glycol, dipropylene glycol,1,4-butanediol, 1,3-butanediol, 2,3-butanediol, 1,2-butanediol,3-methyl-1,2-butanediol, 1,2-pentanediol, 1,5-pentanediol,1,4-pentanediol, 2,4-pentanediol, 2,3-dimethyltrimethylene glycol,tetramethylene glycol, 3-methyl-4,3-pentanediol,3-methyl-4,5-pentanediol, 2,2,4-trimethyl-1,3-pentanediol,1,6-hexanediol, 1,5-hexanediol, 1,4-hexanediol, 2,5-hexanediol,1,4-cyclohexanedimethanol, neopentyl glycol and neopentyl glycol esterof hydroxypivalic acid; polyester diols such as polylactone diolprepared by reacting these glycols with E-caprolactone or like lactones,and bis(hydroxyethyl)terephthalate; polyether diols such as alkyleneoxide addition reaction products of bisphenol A, polyethylene glycol,polypropylene glycol and polybutylene glycol, and like dihydricalcohols. Examples of polyhydric alcohols also include trihydric orhigher polyhydric alcohols such as glycerin, trimethylol propane,trimethylol ethane, diglycerin, triglycerin, 1,2,6-hexanetriol,pentaerythritol, dipentaerythritol, sorbitol and mannitol, polylactonepolyols such as addition reaction products of these polyhydric alcoholswith lactones such as ε-caprolactone; and alicyclic polyhydric alcoholssuch as 1,4-cyclohexanedimethanol, tricyclodecanedimethanol,hydrogenated bisphenol A or hydrogenated bisphenol F, alkylene oxideaddition reaction product of hydrogenated bisphenol A, and alkyleneoxide addition reaction product of hydrogenated bisphenol F. Themonoepoxide compound to be optionally used in combination includes, forexample, propylene oxide, butylene oxide and like α-olefin epoxide, and"Cardula E10" (trade name, product of Shell Chemical Co., Ltd., glycidylester of highly branched unsaturated fatty acid).

The acid component may be a polybasic acid and optionally may be used incombination with monobasic acid. Useful polybasic acids are, forexample, phthalic anhydride, isophthalic acid, tetrahydrophthalicanhydride, hexahydrophthalic anhydride, maleic anhydride, succinicanhydride, adipic acid, sebacic acid, trimellitic anhydride andpyromellitic anhydride. Examples of monobasic acids to be optionallyused in combination are aromatic monobasic acids such as benzoic acid,methylbenzoic acid and p-t-butylbenzoic acid; saturated or unsaturatedfatty acids of 1 to 24 carbon atoms such as formic acid, acetic acid,lactic acid, propionic acid, butyric acid, caproic acid, caprylic acid,pelargonic acid, capric acid, undecanoic acid, lauric acid, myristicacid, palmitic acid, stearic acid, cyclohexanecarboxylic acid,9-decenoic acid, oleic acid, eleostearic acid, elaidic acid, brassidicacid, linoleic acid and linolenic acid; and hydroxycarboxylic acids suchas dimethylolpropionic acid, hydroxypivalic acid, 12-hydroxydodecanoicacid, 12-hydroxystearic acid, ricinoleic acid, para-hydroxybenzoic acid,salicylic acid and 4,4-bis(4'-hydroxyphenyl)pentanoic acid. Also usableas the monobasic acid component are lower alkyl esters of thesemonobasic acids, glycerides and cyclic ester compounds such asε-caprolactone, γ-valerolactone and like lactones.

Examples of the oil component are fats and oils such as castor oil, tungoil, safflower oil, soybean oil, linseed oil, tall oil, coconut oil, andfatty acids of these fats and oils.

(iii) Typical examples of the carboxyl-containing resin also includegraft resins prepared by grafting acrylic resins or vinyl resins topolyester resins such as graft copolymers prepared by polymerizing avinyl monomer and/or an acryl monomer with polyester resins having apolymerizable unsaturated group, these resins preferably having a numberaverage molecular weight of about 5,000 to about 40,000.

The carboxyl-containing resin essentially has an acid value of 5 to 100mgKOH/g. If the acid value is lower than 5 mgKOH/g, the first basecoating composition (A) is incompatible with the second base coatingcomposition (B) on the composition (A) and absorbs water from thecomposition (B) in a lower degree, resulting in problems of flowirregularities and sagging of composition (B) and in degraded estheticproperty and surface smoothness of coating film. Hence it isundesirable. On the other hand, an acid value of more than 100 mgKOH/greduces the water resistance of coating film and is hence undesirable. Apreferred acid value is 10 to 50 mgKOH/g, and a more preferred acidvalue is 30 to 50 mgKOH/g.

The carboxyl-containing resin may incorporate a hydroxyl group. Theresin containing a hydroxyl group improves film properties such ashardness, water resistance and weatherability with advantages. Hydroxylgroups can be easily introduced when a carboxyl-containing polymerizableunsaturated monomer is used conjointly with a hydroxyl-containingpolymerizable unsaturated monomer in the production of the resin in item(i). In view of improved film properties, it is suitable to use a resinhaving a hydroxyl value of preferably about 10 to about 200 mgKOH/g,more preferably 25 to 70 mgKOH/g.

The carboxyl-containing resin is neutralized with a neutralizing agentbefore use. Useful neutralizing agents include, for example, basicsubstances such as ammonia, methylamine, ethylamine, dimethylamine,diethylamine, trimethylamine, triethylamine, dimethylethanolamine,diethanolamine and triethanolamine. The amount of the neutralizing agentto be used is preferably about 0.1 to about 2.0 equivalents, morepreferably about 0.3 to about 1.2 equivalents, per equivalent of thecarboxyl group in the resin. If the amount is less than 0.1 equivalent,the first base coating composition (A) is incompatible with the secondbase coating composition (B) on the composition (A) and absorbs waterfrom the composition (B) in a lower degree, resulting in flowirregularities and sagging of composition (B) and in degraded estheticproperty and surface smoothness of coating film. Hence it isundesirable. On the other hand, an amount of more than 2.0 equivalentsreduces the water resistance of coating film and is hence undesirable.

An amino resin is used as a crosslinking agent for thecarboxyl-containing resin in the first base coating composition (A).

Examples of the amino resin to be used include methylolated amino resinsprepared by a reaction between an amino component and an aldehydecomponent, and such methylolated amino resin etherified with amonohydric alcohol. These amino resins may contain a carboxyl group. Inthe reaction process, the amino component and the aldehyde component aresubjected to a condensation reaction in the conventional manner at analkalinity or acidity using about 0.5 to about 2.0 equivalents ofaldehyde group per equivalent of amino group and a pH adjustor (such asammonia, sodium hydroxide, amines or the like). Examples of the aminocomponent are melamine, urea, benzoguanamine, acetoguanamine,steroguanamine, spiroguanamine, dicyandiamide and the like. Examples ofthe aldehyde component are formaldehyde, paraformaldehyde, acetoaldehydeand benzaldehyde. Examples of the monohydric alcohol are monohydricalcohols having 1 to 8 carbon atoms such as miethyl alcohol, ethylalcohol, n-propyl alcohol, i-propyl alcohol, n-butyl alcohol, i-butylalcohol, 2-ethyl butanol and 2-ethyl hexanol.

Examples of the amino resin to be used in the composition (A) arepreferably etherified melamine resins, e.g. those etherified to anextent of etherifying 3 to 6 methylol groups out of 6 methylol groups.

The proportions of the neutralized carboxyl-containing resin and theamino resin in the composition (A) are about 50 to about 90% by weight,or in view of film properties, preferably about 60 to about 85% byweight, of the former, and about 50 to about 10% by weight, preferablyabout 40 to about 15% by weight, of the latter, based on the combinedweight of the two resins calculated as solids.

The first base coating composition (A) contains a metallic pigment orlike pigment, in addition to the neutralized carboxyl-containing resinand the amino resin, to impart the specific hiding power to the curedmetallic coating film.

Useful metallic pigments are, for example, aluminum powders, bronzepowders, copper powders, tin powders, lead powders, zinc powders, ironphosphate powders, micaceous iron oxide particles and titaniumoxide-coated mica particles. At least one of them can be used.

Optionally the metallic pigment may be used in combination with coloringpigments and/or extender pigments. Examples of useful coloring pigmentsare titanium dioxide, carbon black, Phthalocyanine Blue, PhthalocyanineGreen, Carbazole Violet, anthrapyridine, Azo Orange, FlavanthroneYellow, Isoindoline Yellow, Azo Yellow, Indanthrone Blue,Dibromoanthanthrone Red, Perylene Red, Azo Red, Anthraquinone Red andQuinacridone Red. Examples of useful extender pigments are precipitatedbarium sulfate, barium carbonate, gypsum, clay, silica, white carbon,diatomaceous earth, talc, magnesium carbonate, alumina white and micapowders.

The pigment can be used in a suitable combination according to thespecific substrate-hiding power, the desired color and other factors.Usually it is preferred to use a metallic pigment alone, or a metallicpigment and a coloring pigment in combination.

A suitable amount of the pigment to be used is one which results in 3%or less, preferably 1% or less, of light transmittance in a cured layerof the first base coating composition (A). The amount of the pigment tobe used is variable depending on the type of pigment and can not bespecifically determined. Usually the amount is 1 to 250 parts by weight,preferably 5 to 150 parts by weight, per 100 parts by weight of theresin solids content (total amount of the base resin and thecrosslinking agent resin).

The term "transmittance of light" used herein refers to an average lighttransmittance defined as follows. A coating composition is applied to aglass plate to a thickness of 15 Am when cured. After curing, the coatedglass plate is immersed in warm water at 60 to 70° C. Then the coatingfilm is peeled and dried to give a cured coating film which is used as asample. The sample is exposed to light at a wavelength of 400 to 700 nmand an average light transmittance (%) in the sample is measured usingan autographic spectrophotometer (product of Hitachi, Ltd., "EPS-3TModel").

The first base coating composition (A) may contain various dye(s) whenso required. Desirable dyes are those which are superior in resistanceto light, solubility in water or in an organic solvent and the like.

The first base coating composition (A) is an organic solvent-based oneand is prepared by dissolving or dispersing a neutralizedcarboxyl-containing resin, an amino resin, a metallic pigment and so onin an organic solvent. The first base coating composition (A) preferablyhas a nonvolatile concentration of about 20 to about 70% by weight atthe time of application. The first base coating composition (A) isdesirably a so-called high solid one which has a nonvolatileconcentration adjusted to about 30 to about 70% by weight at the time ofapplication in view of prevention of air pollution and saving ofresources.

The organic solvent to be used in the invention can be any ofconventional organic solvents used for coating compositions and include,for example, ester solvents, ether solvents, alcohol solvents, amidesolvents, ketone solvents, aliphatic hydrocarbon solvents, alicyclichydrocarbon solvents and aromatic hydrocarbon solvents. At least onesolvent selected from them is used. Among them, preferred arehydrophilic organic solvents such as ethylene glycol monomethyl etheracetate, diethylene glycol monomethyl ether acetate, diethylene glycolmonoethyl ether acetate, dioxane, ethylene glycol monomethyl ether,ethylene glycol monoethyl ether, ethylene glycol monobutyl ether,diethylene glycol monomethyl ether, diethylene glycol monoethyl ether,diethylene glycol diethyl ether, diethylene glycol monobutyl ether,methyl alcohol, ethyl alcohol, allyl alcohol, n-propyl alcohol,isopropyl alcohol, t-butyl alcohol, ethylene glycol, 1,2-propyleneglycol, 1,2-propylene glycol monomethyl ether, 1,2-propylene glycolmonoethyl ether, 1,2-propylene glycol monobutyl ether, 1,3-butyleneglycol, 2,3-butylene glycol, hexylene glycol, hexanediol, dipropyleneglycol, dipropylene glycol monomethyl ether, dipropylene glycolmonoethyl ether, dipropylene glycol monobutyl ether, acetone anddiacetone alcohol. These hydrophilic organic solvents are used eitheralone or in combination. The hydrophilic organic solvents to be used arethose which dissolve in an amount of at least 50 parts by weight per 100parts by weight of water at 20° C. The first base coating composition(A) contains the hydrophilic organic solvent in a proportion of at least20% by weight, preferably 40 to 100% by weight, based on the combinedweight of organic solvents at the time of application.

According to the coating method of the invention, the first base coatingcomposition (A) is applied to a metal substrate coated with a cationicelectrodepositable coating composition and optionally an intercoatcomposition. The applicator is preferably any of atomizing coaters suchas air spray coaters, airless spray coaters, air-atomizing electrostaticcoating devices and rotary electrostatic coating devices. Suitably thefirst base composition (A) is adjusted at the time of application to aviscosity of about 15 to about 60 seconds, preferably about 15 to about35 seconds (Ford cup #4 at 20° C.). The film thickness is about 2 toabout 60 pm, preferably about 5 to about 25 pm when cured.

The second base coating composition (B) is applied to the coatingsurface of the first base coating composition (A) layer according to themethod of the invention. The second base coating composition (B) is anaqueous thermosetting coating composition which shows a transparencywhich is commensurate with a light transmittance of 10 to 95% in a curedcoating film. The second base coating composition (B) comprises anaqueous base resin, a crosslinking agent, a pigment and so on. Water isused as a main solvent.

Suitable aqueous base resins as the component for the second basecoating composition (B) are, for example, water-soluble orwater-dispersible acrylic resins, water-soluble or water-dispersiblepolyester resins and the like.

Water-soluble or water-dispersible acrylic resins useful herein includeall of conventional resins. Typical examples of water-soluble acrylicresins are those prepared by polymerizing at least one polymerizablemonomer selected from α,β-ethylenically unsaturated carboxylic acids,hydroxyalkyl esters thereof and other α,β-ethylenically unsaturatedmonomers, the acrylic resin having an acid value of about 20 to about100 mgKOH/g and a hydroxyl value of about 20 to about 200 mgKOH/g. Ifthe acrylic resin has an acid value of less than 20 mgKOH/g, the resinis hard to dissolve in water, whereas the acrylic resin having an acidvalue of more than 100 mgKOH/g reduces the water resistance of coatingfilm due to remaining carboxyl group.

Examples of polymerizable monomers useful in the preparation of thewater-soluble acrylic resin are α,β-ethylenically unsaturated carboxylicacids such as (meth)acrylic acid, maleic acid and itaconic acid;hydroxyalkyl esters of α,β-ethylenically unsaturated carboxylic acidssuch as 2-hydroxyethyl (meth)acrylate and 2-hydroxypropyl(meth)acrylate; alkyl esters of α,β-ethylenically unsaturated carboxylicacids such as methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl(meth)acrylate, isobutyl (meth)acrylate, 2-ethylhexyl (meth)acrylate,lauryl (meth)acrylate and decyl (meth)acrylate; acrylamide derivativessuch as (meth)acrylamide, N-methylol (meth)acrylamide and diacetoneacrylamide; glycidyl esters of α,β-ethylenically unsaturated carboxylicacids such as glycidyl (meth)acrylate; vinyl esters of saturatedcarboxylic acids such as vinyl acetate and vinyl propionate; aromaticunsaturated monomers such as styrene, α-methylstyrene and vinyltoluene.

The polymerization reaction can be carried out using the above-mentionedmonomers in an organic solvent in the presence of a radicalpolymerization initiator such as azobisisobutyronitrile, benzoylperoxide, dibutyl peroxide and the like. The organic solvent to be usedis one which is compatible with water such as butyl cellosolve, ethyleneglycol monobutyl ether, diethylene glycol monoethyl ether, diethyleneglycol monobutyl ether, 1,2-propylene glycol monomethyl ether and thelike.

The water-soluble acrylic resin thus obtained is neutralized with abasic substance such as ammonia, amines such as triethylamine,monoethanolamine, diethanolamine, triethanolamine, dimethylaminoethanoland hydroxides of alkali metals such as sodium hydroxide. Then, water oroptionally an organic solvent compatible with water is added to theresin to dilute the resin to a suitable solids content.

Useful water-dispersible acrylic resins are, for example, conventionalacrylic resins prepared in the conventional manner by emulsionpolymerization of the foregoing polymerizable unsaturated monomers asthe raw material for the water-soluble acrylic resin in an aqueousmedium in the presence of a dispersion stabilizer. Useful dispersionstabilizers are, for example, at least one member selected from thegroup consisting of ionic low-molecular surface active substances, ionichigh-molecular surface active substances, non-ionic low-molecularsurface active substances and non-ionic high-molecular surface activesubstances, and water-soluble resins.

Usable water-soluble or water-dispersible polyester resins include, forexample, polyester resins having an acid value of about 15 to about 100mgKOH/g and a hydroxyl value of about 20 to about 200 mgKOH/g, amongthose described above as the base resin for the first base coatingcomposition (A). The polyester resin is neutralized before use.

An amino resin can be suitably used as the crosslinking agent for thesecond base coating composition (B).

The amino resin to be used can be any of amino resins for use in thefirst base coating composition (A). The amino resin for use in thesecond base coating composition (B) may be an etherified melamine resin,e.g. preferably those etherified to such extent that 2 to 5.5 methylolgroups out of 6 groups have been etherified.

As to the proportions of the aqueous base resin and the amino resin asthe crosslinking agent in the second base coating composition (B), theproportion of the base resin is about 50 to about 90% by weight,preferably about 60 to about 85% by weight, while the proportion of theamino resin is about 50 to about 10% by weight, preferably about 40 toabout 15% by weight.

The second base coating composition (B) preferably contains a metallicpigment, a coloring pigment or like pigments which leads to theformation of cured coating film having a transparency corresponding to alight transmittance of 10 to 95%. Preferably the total amount ofpigments is so small that the layer of the coating composition (B) istransparent with the result that the color of first base coatingcomposition layer can be seen through the second base coatingcomposition layer. Desirably the total amount of pigments in the secondbase coating composition (B) is smaller than in the first base coatingcomposition (A).

Stated more specifically, the pigments are preferably used in an amountwhich brings about a light transmittance of 10 to 95%, preferably 20 to90% in a cured layer of second base coating composition (B). The amountis variable depending on the type of pigments to be used and can not bespecifically determined. Usually the amount is 0.01 to 100 parts byweight, preferably 0.1 to 80 parts by weight, per 100 parts by weight ofthe resin solid (total amount of the base resin and the crosslinkingagent resin), and the amount is preferably smaller than in the firstbase coating composition (A).

The second base coating composition (B) may contain various dyes when sorequired.

The second base coating composition (B) can be applied with use of thesame coater as used in application of the first base coating composition(A). A preferred viscosity of the second base coating composition (B) atthe time of application is about 10 to about 50 seconds in terms of Fordcup #4 (20° C.). The film thickness is about 5 to about 30 μm,preferably about 10 to about 20 μm, when cured.

Desirably the first base and the second base coating compositions (A)and (B) have an affinity for each other. More specifically, the baseresins and the crosslinking agent resins in the coating compositions (A)and (B) are entirely or partly the same as or similar to each other, oreven if the other is of different type, there exists an affinity betweenthe two compositions.

The clear coating composition (C) is applied to the coating surface ofthe second base coating composition (B) layer according to the method ofthe invention. The clear coating composition (C) forms a clear topcoating layer which contributes to improvements in the properties ofmulti-layer film such as esthetic property, finished appearance,weatherability, chemical resistance, water resistance, moistureresistance and so on. The clear coating composition (C) is athermosetting composition comprising a base resin, a crosslinking agentand other components. An organic solvent is used as a solvent in thecomposition (C).

A preferred base resin in the clear coating composition (C) is ahydroxyl-containing acrylic resin. Suitable acrylic resins are, forexample, copolymers prepared by copolymerizing a hydroxyl-containingpolymerizable unsaturated monomer and other polymerizable unsaturatedmonomers. At least one of these monomers is acrylic. It is preferredthat the resin have a number average molecular weight of about 2,000 toabout 100,0000 and a hydroxyl value of about 50 to about 200 mgKOH/g.

Examples of the hydroxyl-containing polymerizable unsaturated monomerare hydroxyalkyl (having 2 to 8 carbon atoms) esters of (meth)acrylicacids such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl(meth)acrylate, 3-hydroxypropyl (meth)acrylate and hydroxybutyl(meth)acrylate; monoesters of polyether polyols such as polyethyleneglycol, polypropylene glycol or polybutylene glycol with unsaturatedcarboxylic acids such as (meth)acrylic acid; monoethers of theabove-mentioned polyether polyols with hydroxyl-containing unsaturatedcompounds such as 2-hydroxyethyl (meth)acrylate; addition reactionproducts of α,β-unsaturated carboxylic acids with monoepoxy compoundssuch as "Cardula E10" (trade name, product of Shell Chemical Co., Ltd.)and a-olefin epoxide; addition reaction products of glycidyl(meth)acrylate with monobasic acids such as acetic acid, propionic acid,p-t-butylbenzoic acid and fatty acids; monoesters or diesters of acidanhydride group-containing unsaturated compounds such as maleicanhydride and itaconic anhydride with glycols such as ethylene glycol,1,6-hexanediol and neopentyl glycol; hydroxyalkyl vinyl ethers such ashydroxyethyl vinyl ether; chlorine- and hydroxyl-containing monomerssuch as 3-chloro-2-hydroxypropyl (meth)acrylate; allyl alcohol and soon. Examples of other polymerizable unsaturated monomers are alkyl orcycloalkyl (having 1 to 24 carbon atoms) esters of (meth)acrylic acidssuch as methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl(meth)acrylate, i-butyl (meth)acrylate, t-butyl (meth)acrylate,cyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl(meth)acrylate, stearyl (meth)acrylate and decyl (meth)acrylate;(meth)acrylamides such as (meth)acrylamide, N-methyl (meth)acrylamide,N-ethyl (meth)acrylamide, diacetone acrylamide, N-methylol(meth)acrylamide or N-butoxymethyl (meth)acrylamide; glycidylgroup-containing vinyl monomers such as glycidyl (meth)acrylate,glycidyl (meth)acrylamide and allyl glycidyl ether; carboxyl-containingvinyl monomers such as (meth)acrylic acid, maleic acid, itaconic acidand crotonic acid; vinyl monomers such as styrene, vinyltoluene, vinylpropionate, a-methyl styrene, vinyl acetate, (meth)acrylonitrile, vinylpivalate and "VeoVa" monomer (trade name, product of Shell Chemical Co.,Ltd.).

An amino resin is preferably used as the crosslinking agent in the clearcoating composition (C). The amino resins exemplified above for thefirst base coating composition (A) can be suitably used.

When required, the clear coating composition (C) may contain additivessuch as defoaming agents, leveling agents and like surface controlagents; thickeners, sagging inhibitors and like rheology control agents;ultraviolet absorbers; light stabilizers and the like.

The clear coating composition (C) is provided in an organic solvent formand can be prepared by dissolving or dispersing the base resin,crosslinking agent and the like in an organic solvent. The clear coatingcomposition (C) is preferably a so-called high solid one which has anonvolatile concentration adjusted to about 35 to about 80% by weight,preferably about 40 to about 80% by weight at the time of application inview of prevention of air pollution and saving of resources.

The organic solvent to be used can be any of conventional organicsolvents used for coating compositions and include, for example, estersolvents, ether solvents, alcohol solvents, amide solvents, ketonesolvents, aliphatic hydrocarbon solvents, alicyclic hydrocarbon solventsand aromatic hydrocarbon solvents. At least one solvent selected fromthem is used.

The clear coating composition (C) can be applied with use of the samecoater as used in application of the first base coating composition (A).A preferred viscosity of the clear coating composition (C) at the timeof application is about 15 to about 60 seconds in terms of Ford cup #4(20° C.). A preferred film thickness is about 5 to about 50 μm whencured.

The method of forming a multi-layer metallic coating film according tothe invention is characterized by a 3-coat system comprising the stepsof applying the first base coating composition (A), the second basecoating composition (B) and the clear coating composition (C) in thisorder over a specific metal substrate. The method of the invention canbe suitably conducted according to a 3-coat 1-bake system or a 3-coat2-bake system as described below.

According to one of preferred embodiments of the invention, the 3-coat1-bake system may be conducted which comprises the steps of applying thefirst base coating composition (A) to the substrate, applying the secondbase coating composition (B) to the first base coating layer withoutcuring the layer, pre-drying the two coating layers at 50 to 100° C.while substantially not curing the two layers, applying the clearcoating composition (C), and heating the 3 coating layers ofcompositions (A), (B) and (C) to cure them at the same time.

In the 3-coat 1-bake system, after application of first base coatingcomposition (A), the coated substrate is properly set and then thesecond base coating composition (B) is applied to the uncured coatinglayer. The uncured two coating layers of coating compositions (A) and(B) are essentially pre-dried at a temperature of 50 to 100° C. afterapplication of coating composition (B) and before application of clearcoating composition (C). Due to this pre-drying, the two coating layersremain substantially free of curing by crosslinking and show a gelfraction of about 5% by weight or less. A suitable pre-drying time isabout 3 to about 30 minutes. Then after the clear coating composition(C) is applied, the three coating layers of coating compositions (A),(B) and (C) are suitably set and heated to about 120 to about 180° C.for about 15 to about 45 minutes for curing by crosslinking at the sametime. In this way, a multi-layer metallic coating film is formed. Thesetting may be conducted as by leaving the coated substrate to stand atroom temperature.

According to another preferred embodiment of the invention, the 3-coat2-bake system may be conducted which comprises the steps of applying thefirst base coating composition (A) to the substrate, applying the secondbase coating composition (B) without curing the first base coatingcomposition layer, heating the two coating layers for curing, applyingthe clear coating composition (C), and heating the coating layer ofcomposition (C) for curing.

More specifically stated, the 3-coat 2-bake system is carried out asfollows. After application of first base coating composition (A), thecoated substrate is properly set, and the second base coatingcomposition (B) is applied to the uncured coating layer. Then afterproper setting, the two coating layers of compositions (A) and (B) areheated at a temperature of about 120 to about 180° C. for about 15 toabout 45 minutes for simultaneous curing by crosslinking. Thereafter theclear coating composition (C) is applied, and then after proper setting,the coating layer of composition (C) is heated at a temperature of about120 to about 180° C. for about 15 to about 45 minutes and cured bycrosslinking. In this way, a multi-layer metallic coating film isformed. The setting may be conducted as by leaving the coated substrateto stand at room temperature.

When the method for forming a multi-layer metallic coating filmaccording to the invention is carried out, a multi-layer metalliccoating film can be formed on a metal substrate, the coating film beingremarkably improved in the depth feeling, distinctness-of-image gloss,brilliant luster and the like, thus enhanced in esthetic property andexcellent in surface smoothness, water resistance and like properties.The method of the invention is feasible with high coating efficiency andcan advantageously overcome the problems on the prevention of airpollution and saving of resources. Further the method of the inventioncan be widely used to coat metal substrates such as motor vehicles,bicycles, electric appliances, etc.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is described below in more detail with referenceto the following preparation examples, examples and comparative exampleswherein the parts and percentages are all by weight.

PREPARATION EXAMPLE 1

(Production of metal substrate)

"Elecron #9400" (trade name for polyamide-modified epoxy resin/blockedpolyisocyanate cationically electrodepositable coating compositionmanufactured by Kansai Paint Co., Ltd.) was diluted with deionized wateror the like to a solid concentration of about 16% to give a cationicallyelectrodepositable coating composition bath.

A steel plate with a surface treated by chemical conversion with"Bonderite #3030" (trade name for a zinc phosphate treating agentmanufactured by Nippon Parker Rising Co., Ltd.) was immersed in theobtained cationically electrodepositable coating composition bath. Thesteel plate held at a pH of 5.5 to 8.0 was coated in the conventionalmanner by cationic electrodeposition. The film thickness was about 20 μmon cured basis. After withdrawal from the bath, the coated steel platewas washed with water and heated to about 175° C. for 20 minutes forcuring. The obtained plate is hereinafter referred to as "substrate I".

The cured surface of electrodeposited layer on substrate I wasair-sprayed with an organic solvent-based intercoat composition "TP-37"(trade name for polyester resin/melamine resin intercoat compositionmanufactured by Kansai Paint Co., Ltd.) adjusted to a viscosity of 20seconds (Ford cup #4/20° C.) to give a coating layer with a thickness ofabout 30 Mm when cured. The coated substrate was left to stand at roomtemperature for about 10 minutes and heated at 140° C. for 30 minutesfor curing. The thus coated substrate is hereinafter referred to as"substrate II".

PREPARATION EXAMPLE 2

(Preparation of carboxyl-containing acrylic resin)

Sixty parts of butyl cellosolve was placed in a reactor and heated to120° C. The following monomer mixture was added using a quantitativepump over a period of 3 hours.

    ______________________________________                                        Methyl methacrylate     30 parts                                              Ethyl acrylate          23 parts                                              n-Butyl acrylate        30 parts                                              Hydroxyethyl methacrylate                                                                             12 parts                                              Acrylic acid             5 parts                                              α,α'-Azobisisobutyronitrile                                                                2 parts                                              ______________________________________                                    

The above procedure gave an acrylic resin solution (a-1) having a resinsolids content of 60%. The acrylic resin had an acid value of 40mgKOH/g, a hydroxyl value of 52 mgKOH/g and a number average molecularweight of about 10,000.

Dimethylaminoethanol (3.5 parts) was added to 100 parts of the acrylicresin solution (a-1) to neutralize 0.95 equivalent. The solution wasdiluted with isopropyl alcohol to give a neutralized acrylic resinsolution (a-1N) having a resin solids content of 50%.

PREPARATION EXAMPLE 3

(Preparation of carboxyl-containing polyester resin)

A reactor was charged with 0.7 mole of neopentyl glycol, 0.3 mole oftrimethylol propane, 0.4 mole of phthalic anhydride and 0.5 mole ofadipic acid. The contents of the reactor were reacted at 200 to 230° C.for 5 hours. After addition of 0.03 mole of trimellitic anhydride, themixture was reacted at 180° C. for a further 1 hour. Butyl cellosolvewas added to give a polyester resin solution (b-1) having a resin solidscontent of 70%. The resin had an acid value of 40 mgKOH/g, a hydroxylvalue of 141 mgKOH/g and a number average molecular weight of about6,000.

Dimethylaminoethanol (4 parts) was added to 100 parts of the polyesterresin solution (b-1) to neutralize 0.9 equivalent. The solution wasdiluted with isopropyl alcohol to give a neutralized polyester resinsolution (b--N) having a resin solids content of 60%.

PREPARATION EXAMPLE 4

(Preparation of first base coating composition (A))

First base coating compositions (A-1) and (A-2) of the invention andcomparative coating compositions (A-3) to (A-5) were prepared using thecomponents listed in Table 1 in the amounts indicated therein.

                  TABLE 1                                                         ______________________________________                                        Coating composition                                                                        A-1     A-2     A-3   A-4   A-5                                  ______________________________________                                        Base resin                                                                    Neutralized acrylic resin                                                                  140             140                                              solution (a-1N)                                                               Acrylic resin solution             117                                        (a-1)                                                                         Neutralized polyester                                                                              117                                                      resin solution (b-1N)                                                         Polyester resin solution                 100                                  (b-1)                                                                         Crosslinking agent                                                                         34      34      34    34    34                                   "Cymel 370"                                                                   Pigment                                                                       "Alupaste 891K"                                                                            20      5       20    20    5                                    "Blue G316"          10                  10                                   "BP-1300"            2                   2                                    Diluting solvent                                                              Isopropyl alcohol                                                                          129     125           140   120                                  Deionized water              180                                              Solids content in coating                                                                  35      40      28    36    43                                   composition (wt %)                                                            Viscosity of coating                                                                       25      25      25    25    25                                   composition (sec.,                                                            Ford cup #4/20° C.)                                                    Light transmittance (%)                                                                    0.1>    0.1>    0.1>  0.1>  0.1>                                 in 15 μm-thick cured                                                       coating film                                                                  ______________________________________                                    

The amounts of the base resins, crosslinking agent, pigments anddiluting solvents used are expressed in "part" in Table 1. "Cymel 370"(trade name, product of Mitsui Cytech Co., Ltd.) in Table 1 is amethylated melamine resin having 1.7 triazine nuclei on the average, anetherification degree of 77% and a solids content of 88%. "Alupaste891K" (trade name, product of Toyo Aluminum Co., Ltd.) is an aluminumpaste having a solids content of 65%. "Blue G316" (trade name, productof Sanyo Shikiso Co., Ltd.) is a blue pigment containing PhthalocyanineBlue as an active ingredient. "BP-1300" (trade name, product of CabbotCo., Ltd.) is a carbon black pigment.

PREPARATION EXAMPLE 5

(Preparation of second base coating composition (B))

(1) A reactor was charged with 140 parts of deionized water, 2.5 partsof 30% "Newcol 707SF" (trade name for a surfactant manufactured byNippon Nyukazai Co., Ltd.) and 1 part of the below-mentioned monomermixture (1). The mixture was mixed with stirring in a nitrogen gasstream. Then, 3 parts of a 3% aqueous solution of ammonium persulfatewas added at 60° C. After the mixture was heated to 80° C., the reactorwas charged with a monomer emulsion comprising 79 parts of the followingmonomer mixture (1), 2.5 parts of 30% "Newcol 707SF", 4 parts of a 3%aqueous solution of ammonium persulfate and 42 parts of deionized waterusing a quantitative pump over a period of 4 hours. After addition, themixture was aged for 1 hour. Then 20.5 parts of the following monomermixture (2) and 4 parts of a 3% aqueous solution of ammonium persulfatewere simultaneously added dropwise at 80° C. to the reactor in aparallel manner over 1.5 hours. After completion of addition, themixture was aged for 1 hour. The aged mixture was diluted with 30 partsof deionized water. The diluted solution was passed through a 200 meshnylon cloth filter at 30° C. Deionized water was added to the filtrateand the filtrate was adjusted to a pH of 7.5 with dimethylaminoethanolto give an acrylic resin emulsion (c-1) having a nonvolatile content of20%. The acrylic resin had an average particle size of 0.1 μm, an acidvalue of 26 mgKOH/g and a hydroxyl value of 24 mgKOH/g.

Monomer mixture (1)

    ______________________________________                                        Methyl methacrylate    55    parts                                            Styrene                8     parts                                            n-Butyl acrylate       9     parts                                            2-Hydroxyethyl acrylate                                                                              5     parts                                            1,6-Hexanediol diacrylate                                                                            2     parts                                            Methacrylic acid       1     part                                             Monomer mixture (2)                                                           Methyl methacrylate    5     parts                                            n-Butyl acrylate       7     parts                                            2-Ethylhexyl acrylate  5     parts                                            Methacrylic acid       3     parts                                            30% "Newcol 707SF"     0.5   part                                             ______________________________________                                    

(2) Second base coating compositions (B-1) and (B-2) and comparativecoating composition (B-3) were prepared using the acrylic resin emulsion(c-1) and the components shown in Table 2 in the amounts indicatedtherein.

                  TABLE 2                                                         ______________________________________                                        Coating composition                                                                            B-1       B-2    B-3                                         ______________________________________                                        Base resin                                                                    Neutralized acrylic resin                                                                      50        50     50                                          solution (a-1N)                                                               Neutralized polyester                                                                          50        50     50                                          resin solution (b-1N)                                                         Acrylic resin emulsion                                                                         100       100    100                                         (c-1)                                                                         Crosslinking agent                                                                             34        34     34                                          "Cymel 370"                                                                   Pigment                                                                       "Alupaste 891K"            2      5                                           "Blue G316"      5                10                                          "BP-1300"                         2                                           Diluting solvent                                                              deionized water  220       220    240                                         Solids content in coating                                                                      25        25     25                                          composition (wt %)                                                            Viscosity of coating compo-                                                                    30        30     30                                          sition (sec., Ford cup                                                        #4/20° C.)                                                             Light transmittance (%)                                                                        40        45     0.1>                                        in 15 μm-thick cured coating                                               film                                                                          ______________________________________                                    

PREPARATION EXAMPLE 6

(Preparation of clear coating composition (C))

(1) Forty parts of "Swasol #1000" (trade name for a hydrocarbon solventmanufactured by Maruzen Oil Co., Ltd.) was placed in a reactor andheated 120° C. To the reactor was added the following monomer mixtureover a period of 3 hours using a quantitative pump.

    ______________________________________                                        Styrene                 30 parts                                              n-Butyl acrylate        35 parts                                              2-Ethylhexyl acrylate   10 parts                                              Hydroxyethyl acrylate   25 parts                                              α,α"-Azobisisobutyronitrile                                                                4 parts                                              ______________________________________                                    

The above procedure gave an acrylic resin solution (a-2) having a resinsolids content of 70%. The acrylic resin had a hydroxyl value of 120mgKOH/g and a number average molecular weight of about 6,000.

(2) A reactor was charged with 58 parts of 60% "Uban 28-60" (trade namefor a butylated melamine resin solution which is 3 in average triazinenucleus number and has an etherification degree of 70%, product ofMitsui Toatsu Chemical Co., Ltd.), 30 parts of n-heptane and 0.15 partof benzoyl peroxide. After the mixture was heated to 95° C., thefollowing monomer mixture was added dropwise over 3 hours.

    ______________________________________                                        Styrene                 15    parts                                           Acrylonitrile           9     parts                                           Methyl methacrylate     13    parts                                           Methyl acrylate         15    parts                                           n-Butyl acrylate        1.8   parts                                           2-Hydroxyethyl methacrylate                                                                           10    parts                                           Acrylic acid            1.2   parts                                           Benzoyl peroxide        0.5   part                                            n-Butanol               5     parts                                           "Shellsol 140"          30    parts                                           n-Heptane               9     parts                                           ______________________________________                                    

One hour after dropwise addition of the monomer mixture, a mixture of0.65 part of t-butyl peroctoate and 3.5 parts of "Shellsol 140" (tradename for hydrocarbon solvent manufactured by Shell Chemical Co., Ltd.)was added dropwise over one hour. The mixture was stirred for 2 hourswhile maintaining the temperature at 95° C. Then, 34 parts of thesolvent was removed under a reduced pressure, giving a nonaqueousdispersion of acrylic resin (d-1) having a resin solids content of 60%and a viscosity of A (Gardner-Holdt bubble viscometer).

(3) A clear coating composition (C-1) was prepared using the followingcomponents in the amounts shown.

    ______________________________________                                        70% acrylic resin solution (a-2)                                                                      57    parts                                           60% nonaqueous dispersion                                                                             50    parts                                           of acrylic resin (d-1)                                                        "Cymel 303"             30    parts                                           25% solution of dodecyl-                                                                              4     parts                                           benzenesulfonic acid                                                          "BYK-300"               0.5   part                                            ______________________________________                                    

The foregoing mixture was adjusted with "Swasol #1000" to a viscosity of30 seconds (Ford cup #4/20° C.), giving a clear coating composition(C-1) with a resin solids content of 55%.

Among said components, "Cymel 303" (trade name, product of Mitsui CytechCo., Ltd.) is a methylated melamine resin having an average triazinenucleus number of 1.2 and an etherification degree of 100%, and"BYK-300" is a leveling agent (trade name, product of Bic Chemie Japan).Examples 1 and 2 and Comparative Examples 1 to 4

Multi-layer metallic coating films were prepared by carrying out thecoating operation described below at 60% RH, 75% RH or 90% RH.

The substrate II obtained in Preparation Example 1 was air-sprayed withany one of the first base coating composition (A-1) or (A-2) and thecomparative coating compositions (A-3) to (A-5) adjusted to a viscosityof 25 seconds (Ford cup #4/20° C.) to give a coating film of about 20 μmthickness (when cured). The coated substrate was left to stand at roomtemperature for about 10 minutes. In Comparative Example 4, the firstbase coating composition was not applied.

The uncured surface of first base coating composition layer wasair-sprayed with any one of second base coating composition (B-1) or(B-2) and comparative coating composition (B-3) adjusted to a viscosityof 30 seconds (Ford cup #4/20° C.) to give a coating film with athickness of about 15 4m when cured. The coated substrate was pre-driedat 80° C. for about 10 minutes.

Subsequently the uncured surface of second base coating compositionlayer was air-sprayed with the clear coating composition (C-1) adjustedto a viscosity of 30 seconds (Ford cup #4/20° C.) to give a coating filmwith a thickness of about 15 μm when cured. The coated substrate wasleft to stand at room temperature for about 10 minutes. Thereafter thecoated substrate was heated to 140° C. for 30 minutes, whereby the threecoating layers of first and second base coating compositions and clearcoating composition were concurrently cured by crosslinking.

Thus, a multi-layer metallic coating film was formed according to the3-coat 1-bake system.

Performance tests were carried out to evaluate the properties of themulti-layer metallic coating films obtained in Examples 1 and 2 andComparative Examples 1 to 4 by the following methods.

Depth feeling: The coated plate was visually inspected and evaluatedaccording to the following criteria.

A: Excellent in depth feeling for notably impressive contrast inlightness between highlight and shade;

B: Good in depth feeling for slightly impressive contrast in lightnessbetween highlight and shade; and

C: Lack of depth feeling for the absence of contrast in lightnessbetween highlight and shade.

Brilliant luster: The coated plate was visually inspected and evaluatedaccording to the following criteria.

A: Excellent in brilliant luster;

B: Good in brilliant luster; and

C: Poor in brilliant luster.

Surface smoothness: The surface smoothness was measured using a tensionmeter (product of Renault, France). The greater the value is, the higherthe surface smoothness is. Distinctness-of-image gloss: Measured with adistinctness-of-image gloss meter "P.G.D-IV" (product of Nihon ShikisaiKenkyusho). The greater the value is, the higher thedistinctness-of-image gloss is.

Water resistance: The coated plate was immersed in warm water at 40° C.for 240 hours, and the state of coating surface was visually evaluatedaccording to the following criteria.

A: No change on the coating surface;

B: Small degrees of dullness, blister or like flaws on the coatingsurface; and

C: Noticeable degrees of dullness, blister or like flaws on the coatingsurface.

The coating compositions used in the tests and the test results areshown in Table 3.

                  TABLE 3                                                         ______________________________________                                                     Example Comparative Example                                                   1    2      1      2    3    4                                   ______________________________________                                        Coating composition used                                                      First base coating comp.                                                                     A-1    A-2    A-3  A-4  A-5  None                              Second base coating comp.                                                                    B-1    B-2    B-1  B-2  B-2  B-3                               Clear coating comp.                                                                          C-1    C-1    C-1  C-1  C-1  C-1                               Film properties                                                               Depth  Humidity 60     A    A    B    B    B    B                             feeling                                                                              (% RH)   75     A    A    C    B    B    C                                             90     A    A    C    C    C    C                             Brilliant                                                                            Humidity 60     A    A    C    B    B    B                             luster (% RH)   75     A    A    C    C    C    C                                             90     A    A    C    C    C    C                             Distinct-                                                                            Humidity 60      1.0  1.0  0.8  0.7  0.8  0.8                          ness-of-                                                                             (% RH)   75      1.0  1.0  0.8  0.7  0.7  0.7                          image           90      1.0  1.0  0.7  0.7  0.7  0.7                          gloss                                                                         Surface                                                                              Humidity 60     18   18   16   17   16   16                            smooth-                                                                              (% RH)   75     18   19   16   17   17   16                            ness            90     19   19   17   17   17   16                            Water  Humidity 60     A    A    B    A    A    A                             resist-                                                                              (% RH)   75     A    A    B    A    A    A                             ance            90     A    A    B    A    A    A                             ______________________________________                                    

EXAMPLES 3 AND 4

Multi-layer metallic coating films were formed at 60% RH, 75% RH or 90%RH by conducting the following coating operation.

The substrate I obtained in Preparation Example 1 was air-sprayed withthe first base coating composition (A-1) or (A-2) adjusted to aviscosity of 25 seconds (Ford cup #4/20° C.) to give a coating film ofabout 20 μm thickness when cured. The coated substrate was left to standat room temperature for about 10 minutes.

Subsequently the uncured surface of first base coating composition layerwas air-sprayed with the second base coating composition (B-1) or (B-2)adjusted to a viscosity of 30 seconds (Ford cup #4/20° C.) to give acoating film with a thickness of about 15 μm when cured. The coatedsubstrate was left to stand at room temperature for about 10 minutes.The coated substrate was heated to 140° C. for 30 minutes so that thetwo coating layers of first and second base coating compositions wereconcurrently cured by crosslinking.

Then, the cured surface of second base coating composition layer wasair-sprayed with the clear coating composition (C-1) adjusted to aviscosity of 30 seconds (Ford cup #4/20° C.) to give a coating film witha thickness of about 15 μm when cured. The coated substrate was left tostand at room temperature for about 10 minutes. Then the coatedsubstrate was heated to 140° C. for 30 minutes for curing bycrosslinking.

Thus, a multi-layer metallic coating film was formed according to the3-coat 2-bake system.

Performance tests were carried out to evaluate the properties of themulti-layer metallic coating films obtained in Examples 3 and 4 by theabove-mentioned methods.

The coating compositions used in the tests and the test results areshown in Table 4.

                  TABLE 4                                                         ______________________________________                                                             Example                                                                       3    4                                                   ______________________________________                                        Coating composition used                                                      First base coating     A-1    A-2                                             comp.                                                                         Second base coating    B-1    B-2                                             comp.                                                                         Clear coating comp.    C-1    C-1                                             Film properties                                                               Depth       Humidity 60        A    A                                         feeling     (% RH)   75        A    A                                                              90        A    A                                         Brilliant   Humidity 60        A    A                                         luster      (% RH)   75        A    A                                                              90        A    A                                         Distinct-   Humidity 60        1.0  1.0                                       ness-of-    (% RH)   75        1.0  1.0                                       image                90        1.0  1.0                                       gloss                                                                         Surface     Humidity 60        18   18                                        smooth-     (% RH)   75        19   19                                        ness                 90        19   19                                        Water       Humidity 60        A    A                                         resis-      (% RH)   75        A    A                                         tance                90        A    A                                         ______________________________________                                    

What is claimed is:
 1. A method for forming a multi-layer metalliccoating film, the method being characterized by a 3-coat systemcomprising the steps of applying the following coating compositions (A),(B) and (C) in this order to a metal substrate coated with acationically electrodepositable coating composition; (A) an organicsolvent-based thermosetting metallic first base coating compositionwhich comprises a neutralized carboxyl-containing resin having an acidvalue of 5 to 100 mgKOH/g, an amino resin and a metallic pigment, thecomposition having a substrate-hiding power corresponding to up to 3%transmittance of light at a wavelength of 400 to 700 nm in a 15 μm-thickcured coating film, (B) an aqueous thermosetting second base coatingcomposition which has a transparency corresponding to 10 to 95%transmittance of light at a wavelength of 400 to 700 nm in a 15-μm thickcured coating film, and (C) an organic solvent-based thermosetting clearcoating composition.
 2. The method according to claim 1 wherein a 3-coat1-bake system is conducted which comprises the steps of applying thefirst base coating composition (A) to the substrate, applying the secondbase coating composition (B) to the layer of first base coatingcomposition (A) without curing the layer, pre-drying the two coatinglayers at 50 to 100° C. while substantially not curing the two layers,applying the clear coating composition (C), and heating the 3 coatinglayers of compositions (A), (B) and (C) to cure them at the same time.3. The method according to claim 1 wherein a 3-coat 2-bake system isconducted which comprises the steps of applying the first base coatingcomposition (A) to the substrate, applying the second base coatingcomposition (B) without curing the layer of first base coatingcomposition (A), heating the two coating layers for curing, applying theclear coating composition (C), and heating the coating layer ofcomposition (C) for curing.
 4. The method according to claim 1 whereinthe first base coating composition (A) contains 1 to 250 parts by weightof the pigment per 100 parts by weight of the resin solids content. 5.The method according to claim 1 wherein the second base coatingcomposition (B) contains 0.01 to 100 parts by weight of the pigment per100 parts by weight of the resin solids content.
 6. An article coated bythe method of claim 1.